This invention relates to thick film circuits and means of fabricating such circuits.
Most thick film circuits presently in use employ a conductor system which is based on the noble metals silver, gold, palladium and platinum, alone or in combination, and a resistor material including ruthenium oxide. Typically, the conductor paste is screen printed onto a ceramic substrate and then fired at temperatures of the order of 950 degrees C. in air. This is followed by screen printing the resistor paste and firing in air at a temperature of approximately 850 degrees C. in air. (See, for example, U.S. Pat. No. 3,998,980 issued to Antes et al.)
Although such circuits are generally satisfactory, several problems exist with the use of precious metal conductor pastes. The primary concern is the rising cost of such precious metals which seriously threatens the economies of thick film technology. In addition, migration of silver across the substrate surface or through dielectrics can cause short circuits thereby restricting use of silver alloy pastes. Furthermore, conductor pastes including silver, gold and palladium can cause poor solderability performance.
As a result of these problems, workers in the art have actively sought alternatives to the precious metal pastes. A copper-based conductor has been considered as one possibility (see, for example, Loasby et al, "Enhanced Property Thick-Film Conductor Pastes," Solid State Technology, May, 1972, p. 46). However, commercially available copper pastes which include glass frits require firing in an atmosphere of nitrogen, and such processing is incompatible with the resistor material. Although it is recognized that copper pastes may be made without glass frits, such materials require a high temperature firing in an oxidizing atmosphere followed by an additional firing in a reducing atmosphere which causes severe degradation of the resistor materials requiring an oxidizing atmosphere.
It is therefore a primary object of the invention to provide a fabrication technique whereby a low cost thick film conductor requiring a reducing atmosphere is made compatible with a thick film resistor requiring an oxidizing atmosphere.